Millimeter wave or terahertz wave band radar is capable of providing small foreign object debris detection and high radar resolution due to its short wavelength and its wide band. For this reason, millimeter wave or terahertz wave band radar is useful as a detection technique for foreign object debris that may intrude into the infrastructure of society. However, in the millimeter wave or terahertz wave band, propagation losses (free space propagation losses that are proportional to the square of the wavelength, and absorption due to water vapor and so on in the atmosphere) are high. Accordingly, even if a single millimeter wave or terahertz wave band radar is used alone, it is difficult to detect foreign object debris over a wide area. In addition, it is not practical to provide a millimeter wave or terahertz wave band radar in each of a number of radar heads, because an expensive and highly precise signal generator is required to detect foreign objects at high accuracy.
A fiber radio (RoF) technique is per se known of superimposing a signal emitted from a high precision signal source arranged in the center upon an optical signal and distributing the result to a number of radar heads via a low loss optical fiber network. Generally, in order to implement a high signal to noise (SN) ratio, a high extinction ratio optical modulator (having a high SN ratio) is needed. It has been considered that a high extinction ratio optical modulator is optimal for generating a high precision millimeter wave or terahertz wave signal, since it is possible to implement a high optical SN ratio by employing a high extinction ratio modulator, because unnecessary sidebands can be suppressed,
For example, with double sideband suppressed carrier modulation (DSB-SC) which implements generation of an optical two-tone signal having optical doubling, the bias voltage of a Mach-Zehnder optical modulator is controlled to the transfer function minimum point. In other words, the bias voltage is controlled so that the optical output becomes minimum when no RF signal is applied. Moreover, for example, with optical quadrupling, in a similar manner, the bias voltage is controlled so that the optical output becomes maximum when no RF signal is applied (refer, for example, to Japanese Patent No. 4,849,621 (Patent Document No. 1 cited below)).
Since a DSB-SC modulation method is used with the optical doubling technique employing an optical modulator, accordingly it has been necessary to perform minimum point modulation for the bias voltage. However since, with a high extinction ratio modulator, the extinction ratio reaches 60 dB or higher, accordingly the allowable range for bias control to yield the optical minimum point and maximum point is extremely narrow, and it has been difficult to perform this control to the minimum point/maximum point with a cheap generic AD converter/control board. In fact, with an adjustable bias voltage of around 12 V, even if control is performed at 0.1 mV or less, sometimes it may happen that the search for the optimum point continues without reaching the minimum point. Furthermore, it also possibly may happen that the point that the control algorithm decides is the minimum point or the maximum point is not the desired control point, but is a local minimum point or maximum point that actually has appeared due to a malfunction of the system or the like. Moreover, in the case of a signal having a high SN ratio such as is required for radar or the like, with a conventional optical modulator having an extinction ratio of around 30 dB, it has been essential to operate at the optical minimum point or maximum point.
A method for adjustment of the optical minimum point and the optical maximum point (i.e., of the bias null point and the bias full point), and a method for evaluation of the characteristics of an optical modulator, are described in Japanese Patent 5,035,411, Japanese Patent 5,137,042, and Japanese Patent 5,354,528, for example.